1. Field of the Invention
The present invention relates to an optical disk recording and reproducing device for recording and reproducing information on an optical disk. In particular, the present invention relates to a control of correcting tilt of an optical pickup with respect to the optical disk.
2. Description of Related Art
Conventional optical disk recording and reproducing devices have an optical pickup that emits a laser beam, which is focused through an objective lens to form a light spot having a diameter of approximately 0.5 μm, for example. Thus, information can be recorded on the optical disk along tracks having a pitch of approximately 0.74 μm, for example, and information recorded along tracks can be reproduced. If the optical axis of the laser beam projected to the surface of the optical disk is not perpendicular to the surface of the optical disk but has some relative tilt error, the light spot may swell to a neighboring track. In this case, information of the neighboring track may be mixed, and aberration may be generated in the light spot on the surface of the optical disk, resulting in substantial deterioration of recording and reproducing quality.
This deterioration of recording and reproducing quality may become a serious problem if the optical disk recording and reproducing device has the optical pickup with a large numerical aperture of the objective lens for realizing high density. Therefore, it is important to control the tilt error of the optical axis of the laser beam with respect to the surface of the optical disk under a predetermined angle. For this purpose, the optical disk recording and reproducing device is required to have a tilt correcting mechanism for controlling the tilt of the optical pickup so as to correct the tilt error in accordance with warp or tilt of the optical disk.
The conventional optical disk recording and reproducing device performs a tilt control of the optical pickup by calculating a maximum amplitude value of an RF signal that is read out from the optical disk if the optical pickup has no tilt sensor, so as to adjust the optical pickup to be a tilt position corresponding to the calculated value. In addition, since an unused optical disk has no RF signal, it is difficult to adjust the optical pickup to be the tilt position corresponding to the maximum amplitude value of the RF signal even if the optical pickup has a variable radial tilt function. Therefore, for an unused optical disk, a tilt adjustment value when a tracking error signal that can be adjusted without the RF signal becomes a maximum amplitude value is usually adopted. However, even if the tilt position of the optical pickup is adjusted at the time point when the tracking error signal becomes the maximum amplitude value, it does not always mean that light to the optical pickup becomes the best state, which can cause deterioration of the recording quality.
For example, JP-A-2004-95035 discloses a conventional technique that utilizes a property that there is a difference of a predetermined offset quantity between the radial tilt correction quantity when the amplitude value of the RF signal read out from the optical disk becomes the maximum and the radial tilt correction quantity when the amplitude value of a land prepit signal becomes the maximum, and the difference does not change regardless of a position on the optical disk, for the tilt correction. In this technique, an optimal correction quantity is determined as the radial tilt correction quantity when the amplitude value of the RF signal becomes the maximum for a used disk, while it is determined for an unused disk by considering the above-mentioned offset quantity that is added to the radial tilt correction quantity when an amplitude value of a land preset signal becomes the maximum.
This patent document says that even an optical disk has a warp, the tilt quantity when the amplitude value of the RF signal becomes the maximum and the tilt quantity when the amplitude value of the land prepit signal becomes the maximum have the same offset regardless of a radial position on the optical disk. However, when information is recorded on an unused disk having a warp, the radial tilt correction quantity corresponding to the radial position when the amplitude value of the land prepit signal at the inner radius of the optical disk becomes the maximum is different from the radial tilt correction quantity corresponding to the radial position when the amplitude value of the land prepit signal at the outer radius of the optical disk becomes the maximum, for example. Nevertheless, this patent document does not disclose a radial position on the optical disk of the radial tilt correction quantity that is used. Therefore, it is not always possible to determine the optimal correction quantity for an unused disk by considering the offset added to the radial tilt correction quantity when the amplitude value of the land preset signal becomes the maximum.
Furthermore, JP-A-2003-16678 discloses another conventional technique for controlling a tilt of an optical pickup. This technique uses a tilt changing portion that changes a tilt quantity between an information surface on the optical disk and a laser beam with respect to a reference angle as a center by changing an angle of the optical pickup with respect to an information quantity, while it calculates an amplitude value of the maximum tracking error signal with respect to a tilt quantity detected by a tilt detecting portion. In accordance with a relationship between the measured tilt quantity and the amplitude value of the tracking error signal, the tilt of the optical pickup is controlled so that the amplitude value of the tracking error signal becomes the maximum.
As the maximum value of the amplitude of the tracking error signal, an average value is used. This average value is obtained by measuring 1/n of a time period necessary for a predetermined number of rotations of the spindle motor, detecting the maximum value of the amplitude of the tracking error signal every n rotations and calculating the average value of the maximum value of the amplitude of the tracking error signal detected n times. Therefore, even if an average value of the maximum value of the amplitude of the tracking error signal is determined by detecting the maximum value of the amplitude of the tracking error signal every n rotations at the inner radius of the optical disk that has a warp, the maximum value of the amplitude of the tracking error signal may be changed at the outer radius in accordance with a state of the warp of the optical disk. Therefore, the determined average value of the maximum value of the amplitude of the tracking error signal is not reliable for a precise tilt control of the optical pickup.
In addition, JP-A-2003-346369 discloses still another conventional technique for a tilt control of an objective lens of the optical pickup. According to this technique, in the case of an unused disk, a warp of an optical disk is detected from an offset of the push-pull tracking error signal. A signal indicating the detected warp is regarded as a tilt correcting signal that is used for the tilt control of an objective lens of the optical pickup. Although the tilt correcting signal is due to the offset of the push-pull tracking error signal, it does not disclose about the RF signal. Therefore, it is not always possible to perform the tilt control such that the amplitude value of the RF signal becomes the maximum.